A CT tube emits x-rays, which is received by one or more detectors facing the tube. The detectors measure an attenuation value of X-rays between the CT tube and the detectors and calculate a distribution of material according to the attenuation value of a detected object measured from every angle of view as well as in coordination with a reconstruction program. Thus images can be reconstructed by the CT system.
Computed tomography reconstruction theory assumes that X-rays emitted from the tube are monochromatic, but this assumption is not the case in a typical system. In fact, the X-rays emitted from a typical system are polychromatic. Therefore, many dark artifacts can appear in reconstructed images obtained by a reconstruction program when an assumption of a monochromatic spectrum is made. These artifacts are referred as beam hardening artifacts. The program used to eliminate these artifacts is referred to as a beam hardening calibration program.
Beam hardening calibration has been the subject of much research around the world. Calibration programs can be divided into two categories according to processing mode: pre-processing and post-processing. Compared with pre-processing calibration methods, post-processing methods for calibration algorithms are more time consuming than pre-processing calibration methods and may cause image information losses, yet there are many post-processing methods emerging in international literature. Pre-processing methods are preferred due to their time-conserving qualities.
The common approach of a pre-processing method includes calculating a beam hardening curve by scanning one or more some special phantoms based upon existing theoretical equations as well as spectrum functions (SF), adjusting the raw data according to this beam hardening curve, and finishing with the beam hardening calibration. During processing, the selection of SF plays an important role on the beam hardening result. The program generally requires two assumptions: (1) that the attenuation detected by the detector in an ideal system should be equal to that of the detected object; and (2) in the ideal system, the X-ray beam emitted from the tube in an optic plane is uniformly distributed for different channels.
For the first assumption, the attenuation detected by the detector in an ideal system should be equal to that of the detected object. In an actual system, the X-ray it will pass through many unknown filter materials before the X-ray arrives at the detector, resulting in the change of beam spectrum. Beam hardening calibration performed according to the spectrum functions (SF) provided by the tube manufacturer may not yield desirable results.
For the second assumption, in an ideal system, the X-ray beam emitted by the tube in an optic plane is uniformly distributed for different channels. Former calibration programs are subject to this assumption and thus deploy consistent calibration parameters (e.g. spectrum function) for different channels during the calibration process. The rays emitted from the tube in an actual system are not uniformly distributed. Thus the attenuation materials that the X-ray beam pass through before arriving at the detected object are also different for different channels, resulting in artifacts if the same beam hardening calibration program is deployed for different channels.
Spectrum functions are an attribute of the tube, thus different tubes correspond to different spectrums. In addition, different scanning parameters correspond to different spectrums. The rays emitted from the tube that pass through a series of filter materials have been changed when they arrive at the detected object. For a different tube, SF is also different for different channels. So SF needs to be adjusted according to real situations.
FIG. 1 shows an example of a prior art system. X-rays emitted from tube 10 pass through filter materials 11-1–11-8 to detector 14. As shown in FIG. 1, SF provided by the tube manufacturer will be altered after it passes through a series of filter materials such as those illustrated by tube target 11-1, glass and metal beryllium of tube export window 11-2, insulating oil 11-3 used to immerse the tube, attached glass layer 11-4, a layer of tungsten on the inner side of the glass sealing 11-5, a first unit for controlling beam width 11-6, a first unit for controlling beam intensity 11-7, attached aluminum and molybdenum 11-8, and others. In addition, the altered SF arrives at the detector only after it passes through the second unit for controlling beam width 13 after passing through detected object 12. Thus, the information detected by the detector 14 includes that of all filter materials 11-1–11-8.